/* -*- mode: C -*-  */
/* vim:set sw=2 ts=2 sts=2 et: */
/* 
   IGraph library.
   Copyright (C) 2007-2012  Gabor Csardi <csardi.gabor@gmail.com>
   334 Harvard street, Cambridge, MA 02139 USA
   
   This program is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2 of the License, or
   (at your option) any later version.
   
   This program is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.
   
   You should have received a copy of the GNU General Public License
   along with this program; if not, write to the Free Software
   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 
   02110-1301 USA

*/

#include "igraph_arpack.h"
#include "igraph_arpack_internal.h"
#include "igraph_memory.h"

#include <math.h>
#include <stdio.h>
#include <string.h>

/* The ARPACK example file dssimp.f is used as a template */

int igraph_i_arpack_err_dsaupd(int error) {
  switch (error) {
  case  1:      return IGRAPH_ARPACK_MAXIT;
  case  3:      return IGRAPH_ARPACK_NOSHIFT;
  case -1:      return IGRAPH_ARPACK_NPOS;
  case -2:      return IGRAPH_ARPACK_NEVNPOS;
  case -3:      return IGRAPH_ARPACK_NCVSMALL;
  case -4:      return IGRAPH_ARPACK_NONPOSI;
  case -5:      return IGRAPH_ARPACK_WHICHINV;
  case -6:      return IGRAPH_ARPACK_BMATINV;
  case -7:      return IGRAPH_ARPACK_WORKLSMALL;
  case -8:      return IGRAPH_ARPACK_TRIDERR;
  case -9:      return IGRAPH_ARPACK_ZEROSTART;
  case -10:     return IGRAPH_ARPACK_MODEINV;
  case -11:     return IGRAPH_ARPACK_MODEBMAT;
  case -12:     return IGRAPH_ARPACK_ISHIFT;
  case -13:     return IGRAPH_ARPACK_NEVBE;
  case -9999:   return IGRAPH_ARPACK_NOFACT;
  default:      return IGRAPH_ARPACK_UNKNOWN;
  }
}

int igraph_i_arpack_err_dseupd(int error) {
  switch (error) {
  case -1:      return IGRAPH_ARPACK_NPOS;
  case -2:      return IGRAPH_ARPACK_NEVNPOS;
  case -3:      return IGRAPH_ARPACK_NCVSMALL;
  case -5:      return IGRAPH_ARPACK_WHICHINV;
  case -6:      return IGRAPH_ARPACK_BMATINV;
  case -7:      return IGRAPH_ARPACK_WORKLSMALL;
  case -8:      return IGRAPH_ARPACK_TRIDERR;
  case -9:      return IGRAPH_ARPACK_ZEROSTART;
  case -10:     return IGRAPH_ARPACK_MODEINV;
  case -11:     return IGRAPH_ARPACK_MODEBMAT;
  case -12:     return IGRAPH_ARPACK_NEVBE;
  case -14:     return IGRAPH_ARPACK_FAILED;
  case -15:     return IGRAPH_ARPACK_HOWMNY;
  case -16:     return IGRAPH_ARPACK_HOWMNYS;
  case -17:     return IGRAPH_ARPACK_EVDIFF;
  default:      return IGRAPH_ARPACK_UNKNOWN;
  }
  
}

int igraph_i_arpack_err_dnaupd(int error) {
  switch (error) {
  case  1:      return IGRAPH_ARPACK_MAXIT;
  case  3:      return IGRAPH_ARPACK_NOSHIFT;
  case -1:      return IGRAPH_ARPACK_NPOS;
  case -2:      return IGRAPH_ARPACK_NEVNPOS;
  case -3:      return IGRAPH_ARPACK_NCVSMALL;
  case -4:      return IGRAPH_ARPACK_NONPOSI;
  case -5:      return IGRAPH_ARPACK_WHICHINV;
  case -6:      return IGRAPH_ARPACK_BMATINV;
  case -7:      return IGRAPH_ARPACK_WORKLSMALL;
  case -8:      return IGRAPH_ARPACK_TRIDERR;
  case -9:      return IGRAPH_ARPACK_ZEROSTART;
  case -10:     return IGRAPH_ARPACK_MODEINV;
  case -11:     return IGRAPH_ARPACK_MODEBMAT;
  case -12:     return IGRAPH_ARPACK_ISHIFT;
  case -9999:   return IGRAPH_ARPACK_NOFACT;
  default:      return IGRAPH_ARPACK_UNKNOWN;
  }
}

int igraph_i_arpack_err_dneupd(int error) {
  switch (error) {
  case  1:      return IGRAPH_ARPACK_REORDER;
  case -1:      return IGRAPH_ARPACK_NPOS;
  case -2:      return IGRAPH_ARPACK_NEVNPOS;
  case -3:      return IGRAPH_ARPACK_NCVSMALL;
  case -5:      return IGRAPH_ARPACK_WHICHINV;
  case -6:      return IGRAPH_ARPACK_BMATINV;
  case -7:      return IGRAPH_ARPACK_WORKLSMALL;
  case -8:      return IGRAPH_ARPACK_SHUR;
  case -9:      return IGRAPH_ARPACK_LAPACK;
  case -10:     return IGRAPH_ARPACK_MODEINV;
  case -11:     return IGRAPH_ARPACK_MODEBMAT;
  case -12:     return IGRAPH_ARPACK_HOWMNYS;
  case -13:     return IGRAPH_ARPACK_HOWMNY;
  case -14:     return IGRAPH_ARPACK_FAILED;
  case -15:     return IGRAPH_ARPACK_EVDIFF;
  default:      return IGRAPH_ARPACK_UNKNOWN;
  }
}

/**
 * \function igraph_arpack_options_init
 * Initialize ARPACK options
 *
 * Initializes ARPACK options, set them to default values.
 * You can always pass the initialized \ref igraph_arpack_options_t
 * object to built-in igraph functions without any modification. The
 * built-in igraph functions modify the options to perform their
 * calculation, e.g. \ref igraph_pagerank() always searches for the
 * eigenvalue with the largest magnitude, regardless of the supplied
 * value.
 * </para><para>
 * If you want to implement your own function involving eigenvalue
 * calculation using ARPACK, however, you will likely need to set up
 * the fields for yourself.
 * \param o The \ref igraph_arpack_options_t object to initialize.
 *
 * Time complexity: O(1).
 */

void igraph_arpack_options_init(igraph_arpack_options_t *o) {
  o->bmat[0]='I';
  o->n=0;			/* needs to be updated! */
  o->which[0]='X'; o->which[1]='X';
  o->nev=1;
  o->tol=0;
  o->ncv=0;         /* 0 means "automatic" */
  o->ldv=o->n;			/* will be updated to (real) n */
  o->ishift=1;
  o->mxiter=3000;
  o->nb=1;
  o->mode=1;
  o->start=0;
  o->lworkl=0;
  o->sigma=0;
  o->sigmai=0;
  o->info=o->start;
  
  o->iparam[0]=o->ishift; o->iparam[1]=0; o->iparam[2]=o->mxiter; o->iparam[3]=o->nb;
  o->iparam[4]=0; o->iparam[5]=0; o->iparam[6]=o->mode; o->iparam[7]=0;
  o->iparam[8]=0; o->iparam[9]=0; o->iparam[10]=0;
}

/**
 * \function igraph_arpack_storage_init
 * Initialize ARPACK storage
 *
 * You only need this function if you want to run multiple eigenvalue
 * calculations using ARPACK, and want to spare the memory
 * allocation/deallocation between each two runs. Otherwise it is safe
 * to supply a null pointer as the \c storage argument of both \ref
 * igraph_arpack_rssolve() and \ref igraph_arpack_rnsolve() to make
 * memory allocated and deallocated automatically.
 *
 * </para><para>Don't forget to call the \ref
 * igraph_arpack_storage_destroy() function on the storage object if
 * you don't need it any more.
 * \param s The \ref igraph_arpack_storage_t object to initialize.
 * \param maxn The maximum order of the matrices.
 * \param maxncv The maximum NCV parameter intended to use.
 * \param maxldv The maximum LDV parameter intended to use.
 * \param symm Whether symmetric or non-symmetric problems will be
 *    solved using this \ref igraph_arpack_storage_t. (You cannot use
 *    the same storage both with symmetric and non-symmetric solvers.)
 * \return Error code.
 *
 * Time complexity: O(maxncv*(maxldv+maxn)).
 */

int igraph_arpack_storage_init(igraph_arpack_storage_t *s, long int maxn,
			       long int maxncv, long int maxldv,
			       igraph_bool_t symm) {
  
  /* TODO: check arguments */
  s->maxn=(int) maxn;
  s->maxncv=(int) maxncv;
  s->maxldv=(int) maxldv;

#define CHECKMEM(x) \
    if (!x) { \
      IGRAPH_ERROR("Cannot allocate memory for ARPACK", IGRAPH_ENOMEM); \
    } \
    IGRAPH_FINALLY(igraph_free, x);

  s->v=igraph_Calloc(maxldv * maxncv, igraph_real_t); CHECKMEM(s->v);
  s->workd=igraph_Calloc(3*maxn, igraph_real_t); CHECKMEM(s->workd);
  s->d=igraph_Calloc(2*maxncv, igraph_real_t); CHECKMEM(s->d);
  s->resid=igraph_Calloc(maxn, igraph_real_t); CHECKMEM(s->resid);
  s->ax=igraph_Calloc(maxn, igraph_real_t); CHECKMEM(s->ax);
  s->select=igraph_Calloc(maxncv, int); CHECKMEM(s->select);

  if (symm) {
    s->workl=igraph_Calloc(maxncv*(maxncv+8), igraph_real_t); CHECKMEM(s->workl);
    s->di=0;
    s->workev=0;
  } else {
    s->workl=igraph_Calloc(3*maxncv*(maxncv+2), igraph_real_t); CHECKMEM(s->workl);
    s->di=igraph_Calloc(2*maxncv, igraph_real_t); CHECKMEM(s->di);
    s->workev=igraph_Calloc(3*maxncv, igraph_real_t); CHECKMEM(s->workev);
    IGRAPH_FINALLY_CLEAN(2);
  }

#undef CHECKMEM
  
  IGRAPH_FINALLY_CLEAN(7);
  return 0;
}

/**
 * \function igraph_arpack_storage_destroy
 * Deallocate ARPACK storage
 *
 * \param s The \ref igraph_arpack_storage_t object for which the
 *    memory will be deallocated.
 *
 * Time complexity: operating system dependent.
 */

void igraph_arpack_storage_destroy(igraph_arpack_storage_t *s) {
  
  if (s->di) {
    igraph_Free(s->di);
  }
  if (s->workev) {
    igraph_Free(s->workev);
  }
  
  igraph_Free(s->workl);
  igraph_Free(s->select);
  igraph_Free(s->ax);
  igraph_Free(s->resid);
  igraph_Free(s->d);
  igraph_Free(s->workd);
  igraph_Free(s->v);
}

/**
 * "Solver" for 1x1 eigenvalue problems since ARPACK sometimes blows up with
 * these.
 */
int igraph_i_arpack_rssolve_1x1(igraph_arpack_function_t *fun, void *extra,
        igraph_arpack_options_t* options,
        igraph_vector_t* values, igraph_matrix_t* vectors) {
  igraph_real_t a, b;
  int nev = options->nev;

  if (nev <= 0) {
    IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_NEVNPOS);
  }

  /* Probe the value in the matrix */
  a = 1;
  if (fun(&b, &a, 1, extra)) {
	  IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
  }

  options->nconv=nev;

  if (values != 0) {
    IGRAPH_CHECK(igraph_vector_resize(values, 1));
    VECTOR(*values)[0] = b;
  }

  if (vectors != 0) {
    IGRAPH_CHECK(igraph_matrix_resize(vectors, 1, 1));
    MATRIX(*vectors, 0, 0) = 1;
  }

  return IGRAPH_SUCCESS;
}

/**
 * "Solver" for 1x1 eigenvalue problems since ARPACK sometimes blows up with
 * these.
 */
int igraph_i_arpack_rnsolve_1x1(igraph_arpack_function_t *fun, void *extra,
        igraph_arpack_options_t* options,
        igraph_matrix_t* values, igraph_matrix_t* vectors) {
  igraph_real_t a, b;
  int nev = options->nev;

  if (nev <= 0) {
    IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_NEVNPOS);
  }

  /* Probe the value in the matrix */
  a = 1;
  if (fun(&b, &a, 1, extra)) {
	  IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
  }

  options->nconv=nev;

  if (values != 0) {
    IGRAPH_CHECK(igraph_matrix_resize(values, 1, 2));
    MATRIX(*values, 0, 0) = b; MATRIX(*values, 0, 1) = 0;
  }

  if (vectors != 0) {
    IGRAPH_CHECK(igraph_matrix_resize(vectors, 1, 1));
    MATRIX(*vectors, 0, 0) = 1; 
  }

  return IGRAPH_SUCCESS;
}

/**
 * "Solver" for 2x2 nonsymmetric eigenvalue problems since ARPACK sometimes
 * blows up with these.
 */
int igraph_i_arpack_rnsolve_2x2(igraph_arpack_function_t *fun, void *extra,
        igraph_arpack_options_t* options, igraph_matrix_t* values,
        igraph_matrix_t* vectors) {
  igraph_real_t vec[2], mat[4];
  igraph_real_t a, b, c, d;
  igraph_real_t trace, det, tsq4_minus_d;
  igraph_complex_t eval1, eval2;
  igraph_complex_t evec1[2], evec2[2];
  igraph_bool_t swap_evals = 0;
  igraph_bool_t complex_evals = 0;
  int nev = options->nev;

  if (nev <= 0) {
    IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_NEVNPOS);
  }
  if (nev > 2)
    nev = 2;

  /* Probe the values in the matrix */
  vec[0] = 1; vec[1] = 0;
  if (fun(mat, vec, 2, extra)) {
	  IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
  }
  vec[0] = 0; vec[1] = 1;
  if (fun(mat+2, vec, 2, extra)) {
	  IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
  }
  a = mat[0]; b = mat[2]; c = mat[1]; d = mat[3];

  /* Get the trace and the determinant */
  trace = a+d;
  det = a*d - b*c;
  tsq4_minus_d = trace*trace / 4 - det;

  /* Calculate the eigenvalues */
  complex_evals = tsq4_minus_d < 0;
  eval1 = igraph_complex_sqrt_real(tsq4_minus_d);
  if (complex_evals) {
    eval2 = igraph_complex_mul_real(eval1, -1);
  } else {
    /* to avoid having -0 in the imaginary part */
    eval2 = igraph_complex(-IGRAPH_REAL(eval1), 0);
  }
  eval1 = igraph_complex_add_real(eval1, trace/2);
  eval2 = igraph_complex_add_real(eval2, trace/2);

  if (c != 0) {
    evec1[0] = igraph_complex_sub_real(eval1, d);
    evec1[1] = igraph_complex(c, 0);
    evec2[0] = igraph_complex_sub_real(eval2, d);
    evec2[1] = igraph_complex(c, 0);
  } else if (b != 0) {
    evec1[0] = igraph_complex(b, 0);
    evec1[1] = igraph_complex_sub_real(eval1, a);
    evec2[0] = igraph_complex(b, 0);
    evec2[1] = igraph_complex_sub_real(eval2, a);
  } else {
    evec1[0] = igraph_complex(1, 0);
    evec1[1] = igraph_complex(0, 0);
    evec2[0] = igraph_complex(0, 0);
    evec2[1] = igraph_complex(1, 0);
  }

  /* Sometimes we have to swap eval1 with eval2 and evec1 with eval2;
   * determine whether we have to do it now */
  if (options->which[0] == 'S') {
    if (options->which[1] == 'M') {
      /* eval1 must be the one with the smallest magnitude */
      swap_evals = (igraph_complex_mod(eval1) > igraph_complex_mod(eval2));
    } else if (options->which[1] == 'R') {
      /* eval1 must be the one with the smallest real part */
      swap_evals = (IGRAPH_REAL(eval1) > IGRAPH_REAL(eval2));
    } else if (options->which[1] == 'I') {
      /* eval1 must be the one with the smallest imaginary part */
      swap_evals = (IGRAPH_IMAG(eval1) > IGRAPH_IMAG(eval2));
    } else {
      IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_WHICHINV);
    }
  } else if (options->which[0] == 'L') {
    if (options->which[1] == 'M') {
      /* eval1 must be the one with the largest magnitude */
      swap_evals = (igraph_complex_mod(eval1) < igraph_complex_mod(eval2));
    } else if (options->which[1] == 'R') {
      /* eval1 must be the one with the largest real part */
      swap_evals = (IGRAPH_REAL(eval1) < IGRAPH_REAL(eval2));
    } else if (options->which[1] == 'I') {
      /* eval1 must be the one with the largest imaginary part */
      swap_evals = (IGRAPH_IMAG(eval1) < IGRAPH_IMAG(eval2));
    } else {
      IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_WHICHINV);
    }
  } else if (options->which[0] == 'X' && options->which[1] == 'X') {
    /* No preference on the ordering of eigenvectors */
  } else {
    /* fprintf(stderr, "%c%c\n", options->which[0], options->which[1]); */
    IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_WHICHINV);
  }

  options->nconv=nev;

  if (swap_evals) {
    igraph_complex_t dummy;
    dummy = eval1; eval1 = eval2; eval2 = dummy;
    dummy = evec1[0]; evec1[0] = evec2[0]; evec2[0] = dummy;
    dummy = evec1[1]; evec1[1] = evec2[1]; evec2[1] = dummy;
  }

  if (complex_evals) {
    /* The eigenvalues are conjugate pairs, so we store only the
     * one with positive imaginary part */
    if (IGRAPH_IMAG(eval1) < 0) {
      eval1 = eval2;
      evec1[0] = evec2[0]; evec1[1] = evec2[1];
    }
  }

  if (values != 0) {
    IGRAPH_CHECK(igraph_matrix_resize(values, nev, 2));
    MATRIX(*values, 0, 0) = IGRAPH_REAL(eval1);
    MATRIX(*values, 0, 1) = IGRAPH_IMAG(eval1);
    if (nev > 1) {
      MATRIX(*values, 1, 0) = IGRAPH_REAL(eval2);
      MATRIX(*values, 1, 1) = IGRAPH_IMAG(eval2);
    }
  }

  if (vectors != 0) {
    if (complex_evals) {
      IGRAPH_CHECK(igraph_matrix_resize(vectors, 2, 2));
      MATRIX(*vectors, 0, 0) = IGRAPH_REAL(evec1[0]);
      MATRIX(*vectors, 1, 0) = IGRAPH_REAL(evec1[1]);
      MATRIX(*vectors, 0, 1) = IGRAPH_IMAG(evec1[0]);
      MATRIX(*vectors, 1, 1) = IGRAPH_IMAG(evec1[1]);
    } else {
      IGRAPH_CHECK(igraph_matrix_resize(vectors, 2, nev));
      MATRIX(*vectors, 0, 0) = IGRAPH_REAL(evec1[0]);
      MATRIX(*vectors, 1, 0) = IGRAPH_REAL(evec1[1]);
      if (nev > 1) {
        MATRIX(*vectors, 0, 1) = IGRAPH_REAL(evec2[0]);
        MATRIX(*vectors, 1, 1) = IGRAPH_REAL(evec2[1]);
      }
    }
  }

  return IGRAPH_SUCCESS;
}

/**
 * "Solver" for symmetric 2x2 eigenvalue problems since ARPACK sometimes blows
 * up with these.
 */
int igraph_i_arpack_rssolve_2x2(igraph_arpack_function_t *fun, void *extra,
        igraph_arpack_options_t* options, igraph_vector_t* values,
        igraph_matrix_t* vectors) {
  igraph_real_t vec[2], mat[4];
  igraph_real_t a, b, c, d;
  igraph_real_t trace, det, tsq4_minus_d;
  igraph_real_t eval1, eval2;
  int nev = options->nev;

  if (nev <= 0) {
    IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_NEVNPOS);
  }
  if (nev > 2)
    nev = 2;

  /* Probe the values in the matrix */
  vec[0] = 1; vec[1] = 0;
  if (fun(mat, vec, 2, extra)) {
	  IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
  }
  vec[0] = 0; vec[1] = 1;
  if (fun(mat+2, vec, 2, extra)) {
	  IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
  }
  a = mat[0]; b = mat[2]; c = mat[1]; d = mat[3];

  /* Get the trace and the determinant */
  trace = a+d;
  det = a*d - b*c;
  tsq4_minus_d = trace*trace / 4 - det;

  if (tsq4_minus_d >= 0) {
    /* Both eigenvalues are real */
    eval1 = trace/2 + sqrt(tsq4_minus_d);
    eval2 = trace/2 - sqrt(tsq4_minus_d);
    if (c != 0) {
      mat[0] = eval1-d; mat[2] = eval2-d;
      mat[1] = c;       mat[3] = c;
    } else if (b != 0) {
      mat[0] = b;       mat[2] = b;
      mat[1] = eval1-a; mat[3] = eval2-a;
    } else {
      mat[0] = 1; mat[2] = 0;
      mat[1] = 0; mat[3] = 1;
    }
  } else {
    /* Both eigenvalues are complex. Should not happen with symmetric
     * matrices. */
    IGRAPH_ERROR("ARPACK error, 2x2 matrix is not symmetric", IGRAPH_EINVAL);
  }

  /* eval1 is always the larger eigenvalue. If we want the smaller
   * one, we have to swap eval1 with eval2 and also the columns of mat */
  if (options->which[0] == 'S') {
    trace = eval1; eval1 = eval2; eval2 = trace;
    trace = mat[0]; mat[0] = mat[2]; mat[2] = trace;
    trace = mat[1]; mat[1] = mat[3]; mat[3] = trace;
  } else if (options->which[0] == 'L' || options->which[0] == 'B') {
    /* Nothing to do here */
  } else if (options->which[0] == 'X' && options->which[1] == 'X') {
    /* No preference on the ordering of eigenvectors */
  } else {
    IGRAPH_ERROR("ARPACK error", IGRAPH_ARPACK_WHICHINV);
  }

  options->nconv=nev;

  if (values != 0) {
    IGRAPH_CHECK(igraph_vector_resize(values, nev));
    VECTOR(*values)[0] = eval1;
    if (nev > 1) {
      VECTOR(*values)[1] = eval2;
    }
  }

  if (vectors != 0) {
    IGRAPH_CHECK(igraph_matrix_resize(vectors, 2, nev));
    MATRIX(*vectors, 0, 0) = mat[0];
    MATRIX(*vectors, 1, 0) = mat[1];
    if (nev > 1) {
      MATRIX(*vectors, 0, 1) = mat[2];
      MATRIX(*vectors, 1, 1) = mat[3];
    }
  }

  return IGRAPH_SUCCESS;
}

int igraph_arpack_rssort(igraph_vector_t *values, igraph_matrix_t *vectors,
			 const igraph_arpack_options_t *options, 
			 igraph_real_t *d, const igraph_real_t *v) {

  igraph_vector_t order;
  char sort[2];
  int apply=1;
  unsigned int n=(unsigned int) options->n;
  int nconv=options->nconv;
  int nev=options->nev;
  unsigned int nans= (unsigned int) (nconv < nev ? nconv : nev);

#define which(a,b) (options->which[0]==a && options->which[1]==b)

  if (which('L','A')) {
    sort[0]='S'; sort[1]='A';
  } else if (which('S','A')) {
    sort[0]='L'; sort[1]='A';
  } else if (which('L','M')) {
    sort[0]='S'; sort[1]='M';
  } else if (which('S','M')) {
    sort[0]='L'; sort[1]='M';
  } else if (which('B','E')) {
    sort[0]='L'; sort[1]='A';
  }

  IGRAPH_CHECK(igraph_vector_init_seq(&order, 0, nconv-1));
  IGRAPH_FINALLY(igraph_vector_destroy, &order);
#ifdef HAVE_GFORTRAN
  igraphdsortr_(sort, &apply, &nconv, d, VECTOR(order), /*which_len=*/ 2);
#else
  igraphdsortr_(sort, &apply, &nconv, d, VECTOR(order));
#endif

  /* BE is special */
  if (which('B','E')) {
    int w=0, l1=0, l2=nev-1;
    igraph_vector_t order2, d2;
    IGRAPH_VECTOR_INIT_FINALLY(&order2, nev);
    IGRAPH_VECTOR_INIT_FINALLY(&d2, nev);
    while (l1 <= l2) {
      VECTOR(order2)[w] = VECTOR(order)[l1]; 
      VECTOR(d2)[w]=d[l1]; 
      w++; l1++;
      if (l1 <= l2) {
	VECTOR(order2)[w] = VECTOR(order)[l2]; 
	VECTOR(d2)[w]=d[l2]; 
	w++; l2--;
      }
    }
    igraph_vector_update(&order, &order2);
    igraph_vector_copy_to(&d2, d);
    igraph_vector_destroy(&order2);
    igraph_vector_destroy(&d2);
    IGRAPH_FINALLY_CLEAN(2);  
  }

#undef which

  /* Copy values */
  if (values) { 
    IGRAPH_CHECK(igraph_vector_resize(values, nans));
    memcpy(VECTOR(*values), d, sizeof(igraph_real_t) * nans);
  }

  /* Reorder vectors */
  if (vectors) {
    int i;
    IGRAPH_CHECK(igraph_matrix_resize(vectors, n, nans));
    for (i=0; i<nans; i++) { 
      unsigned int idx=(unsigned int) VECTOR(order)[i];
      const igraph_real_t *ptr=v + n * idx;
      memcpy(&MATRIX(*vectors, 0, i), ptr, sizeof(igraph_real_t) * n);
    }
  }
  
  igraph_vector_destroy(&order);
  IGRAPH_FINALLY_CLEAN(1);

  return 0;
}

int igraph_arpack_rnsort(igraph_matrix_t *values, igraph_matrix_t *vectors,
			 const igraph_arpack_options_t *options, 
			 igraph_real_t *dr, igraph_real_t *di, 
			 igraph_real_t *v) {

  igraph_vector_t order;
  char sort[2];
  int apply=1;
  unsigned int n=(unsigned int) options->n;
  int nconv=options->nconv;
  int nev=options->nev;
  unsigned int nans=(unsigned int) (nconv < nev ? nconv : nev);

#define which(a,b) (options->which[0]==a && options->which[1]==b)

  if (which('L','M')) {
    sort[0]='S'; sort[1]='M';
  } else if (which('S', 'M')) {
    sort[0]='L'; sort[1]='M';
  } else if (which('L', 'R')) {
    sort[0]='S'; sort[1]='R';
  } else if (which('S', 'R')) {
    sort[0]='L'; sort[1]='R';
  } else if (which('L', 'I')) {
    sort[0]='S'; sort[1]='I';
  } else if (which('S', 'I')) {
    sort[0]='L'; sort[1]='I';
  }

#undef which
  
  IGRAPH_CHECK(igraph_vector_init_seq(&order, 0, nconv-1));
  IGRAPH_FINALLY(igraph_vector_destroy, &order);
#ifdef HAVE_GFORTRAN
  igraphdsortc_(sort, &apply, &nconv, dr, di, VECTOR(order), /*which_len=*/ 2);
#else
  igraphdsortc_(sort, &apply, &nconv, dr, di, VECTOR(order));
#endif

  if (values) {
    IGRAPH_CHECK(igraph_matrix_resize(values, nans, 2));
    memcpy(&MATRIX(*values, 0, 0), dr, sizeof(igraph_real_t) * nans);
    memcpy(&MATRIX(*values, 0, 1), di, sizeof(igraph_real_t) * nans);
  }

  if (vectors) {
    int i, nc=0, nr=0, ncol, wh=0, vx=0;
    for (i=0; i<nans; i++) {
      if (di[i] == 0) { nr++; } else { nc++; }
    }
    ncol=(nc/2)*2 + (nc%2)*2 + nr;
    IGRAPH_CHECK(igraph_matrix_resize(vectors, n, ncol));
    for (i=0; i<nans; i++) {
      unsigned int idx=(unsigned int) VECTOR(order)[i];
      igraph_real_t *ptr=v + n * idx;
      if (di[i]==0) {
	memcpy(&MATRIX(*vectors, 0, vx), ptr, sizeof(igraph_real_t) * n);
	vx++;
      } else if (wh==0) {
	if (di[i] < 0) { ptr -= n; }
	memcpy(&MATRIX(*vectors, 0, vx), ptr, sizeof(igraph_real_t) * n * 2);
	wh=1-wh;
	vx+=2;
      } else {
	wh=1-wh;
      }
    }
  }

  igraph_vector_destroy(&order);
  IGRAPH_FINALLY_CLEAN(1);

  return 0;
}

/**
 * \function igraph_i_arpack_auto_ncv
 * \brief Tries to set up the value of \c ncv in an \c igraph_arpack_options_t
 *        automagically.
 */
void igraph_i_arpack_auto_ncv(igraph_arpack_options_t* options) {
  /* This is similar to how Octave determines the value of ncv, with some
   * modifications. */
  int min_ncv = options->nev * 2 + 1;

  /* Use twice the number of desired eigenvectors plus one by default */
  options->ncv = min_ncv;
  /* ...but use at least 20 Lanczos vectors... */
  if (options->ncv < 20) {
    options->ncv = 20;
  }
  /* ...but having ncv close to n leads to some problems with small graphs
   * (example: PageRank of "A <--> C, D <--> E, B"), so we don't let it
   * to be larger than n / 2...
   */
  if (options->ncv > options->n / 2) {
    options->ncv = options->n / 2;
  }
  /* ...but we need at least min_ncv. */
  if (options->ncv < min_ncv) {
    options->ncv = min_ncv;
  }
  /* ...but at most n-1 */
  if (options->ncv > options->n) {
    options->ncv = options->n;
  }
}

/**
 * \function igraph_i_arpack_report_no_convergence
 * \brief Prints a warning that informs the user that the ARPACK solver
 *        did not converge.
 */
void igraph_i_arpack_report_no_convergence(const igraph_arpack_options_t* options) {
  char buf[1024];
  snprintf(buf, sizeof(buf), "ARPACK solver failed to converge (%d iterations, "
      "%d/%d eigenvectors converged)", options->iparam[2],
      options->iparam[4], options->nev);
  IGRAPH_WARNING(buf);
}

/**
 * \function igraph_arpack_rssolve
 * \brief ARPACK solver for symmetric matrices
 *
 * This is the ARPACK solver for symmetric matrices. Please use
 * \ref igraph_arpack_rnsolve() for non-symmetric matrices.
 * \param fun Pointer to an \ref igraph_arpack_function_t object,
 *     the function that performs the matrix-vector multiplication.
 * \param extra An extra argument to be passed to \c fun.
 * \param options An \ref igraph_arpack_options_t object.
 * \param storage An \ref igraph_arpack_storage_t object, or a null
 *     pointer. In the latter case memory allocation and deallocation
 *     is performed automatically. Either this or the \p vectors argument 
 *     must be non-null if the ARPACK iteration is started from a 
 *     given starting vector. If both are given \p vectors take 
 *     precedence.
 * \param values If not a null pointer, then it should be a pointer to an
 *     initialized vector. The eigenvalues will be stored here. The
 *     vector will be resized as needed.
 * \param vectors If not a null pointer, then it must be a pointer to
 *     an initialized matrix. The eigenvectors will be stored in the
 *     columns of the matrix. The matrix will be resized as needed.
 *     Either this or the \p vectors argument must be non-null if the
 *     ARPACK iteration is started from a given starting vector. If
 *     both are given \p vectors take precedence.
 * \return Error code.
 *
 * Time complexity: depends on the matrix-vector
 * multiplication. Usually a small number of iterations is enough, so
 * if the matrix is sparse and the matrix-vector multiplication can be
 * done in O(n) time (the number of vertices), then the eigenvalues
 * are found in O(n) time as well.
 */

int igraph_arpack_rssolve(igraph_arpack_function_t *fun, void *extra,
			  igraph_arpack_options_t *options,
			  igraph_arpack_storage_t *storage,
			  igraph_vector_t *values, igraph_matrix_t *vectors) {
  
  igraph_real_t *v, *workl, *workd, *d, *resid, *ax;
  igraph_bool_t free_them=0;
  int *select, i;

  int ido=0;
  int rvec= vectors || storage ? 1 : 0;	/* calculate eigenvectors? */
  char *all="All";

  int origldv=options->ldv, origlworkl=options->lworkl,
    orignev=options->nev, origncv=options->ncv;
  char origwhich[2]={ options->which[0], options->which[1] };
  igraph_real_t origtol=options->tol;

  /* Special case for 1x1 and 2x2 matrices */
  if (options->n == 1) {
    return igraph_i_arpack_rssolve_1x1(fun, extra, options, values, vectors);
  } else if (options->n == 2) {
    return igraph_i_arpack_rssolve_2x2(fun, extra, options, values, vectors);
  }

  /* Brush up options if needed */
  if (options->ldv == 0) { options->ldv=options->n; }
  if (options->ncv == 0) {
		igraph_i_arpack_auto_ncv(options);
  }
  if (options->lworkl == 0) { options->lworkl=options->ncv*(options->ncv+8); }
  if (options->which[0] == 'X') { options->which[0]='L'; options->which[1]='M'; }
  
  if (storage) {
    /* Storage provided */
    if (storage->maxn < options->n) {
      IGRAPH_ERROR("Not enough storage for ARPACK (`n')", IGRAPH_EINVAL);
    }
    if (storage->maxncv < options->ncv) {
      IGRAPH_ERROR("Not enough storage for ARPACK (`ncv')", IGRAPH_EINVAL);
    }
    if (storage->maxldv < options->ldv) {
      IGRAPH_ERROR("Not enough storage for ARPACK (`ldv')", IGRAPH_EINVAL);
    }

    v      = storage->v;
    workl  = storage->workl;
    workd  = storage->workd;
    d      = storage->d;
    resid  = storage->resid;
    ax     = storage->ax;
    select = storage->select;
    
  } else {
    /* Storage not provided */
    free_them=1;
    
#define CHECKMEM(x) \
    if (!x) { \
      IGRAPH_ERROR("Cannot allocate memory for ARPACK", IGRAPH_ENOMEM); \
    } \
    IGRAPH_FINALLY(igraph_free, x);

    v=igraph_Calloc(options->ldv * options->ncv, igraph_real_t); CHECKMEM(v);
    workl=igraph_Calloc(options->lworkl, igraph_real_t); CHECKMEM(workl);
    workd=igraph_Calloc(3*options->n, igraph_real_t); CHECKMEM(workd);
    d=igraph_Calloc(2*options->ncv, igraph_real_t); CHECKMEM(d);
    resid=igraph_Calloc(options->n, igraph_real_t); CHECKMEM(resid);
    ax=igraph_Calloc(options->n, igraph_real_t); CHECKMEM(ax);
    select=igraph_Calloc(options->ncv, int); CHECKMEM(select);

#undef CHECKMEM

  }

  /* Set final bits */
  options->iparam[0]=options->ishift;
  options->iparam[2]=options->mxiter;
  options->iparam[3]=options->nb;
  options->iparam[4]=0;
  options->iparam[6]=options->mode;
  options->info=options->start;
  if (options->start) {
    if (!storage && !vectors) {
      IGRAPH_ERROR("Starting vector not given", IGRAPH_EINVAL);
    }
    if (vectors && (igraph_matrix_nrow(vectors) != options->n || 
		    igraph_matrix_ncol(vectors) != 1)) {
      IGRAPH_ERROR("Invalid starting vector size", IGRAPH_EINVAL);
    }
    if (vectors) {
      for (i=0; i<options->n; i++) {
	resid[i]=MATRIX(*vectors, i, 0);
      }
    }
  }

  /* Ok, we have everything */
  while (1) {
#ifdef HAVE_GFORTRAN
    igraphdsaupd_(&ido, options->bmat, &options->n, options->which,
		  &options->nev, &options->tol,
		  resid, &options->ncv, v, &options->ldv,
		  options->iparam, options->ipntr,
		  workd, workl, &options->lworkl, &options->info,
		  /*bmat_len=*/ 1, /*which_len=*/ 2);
#else
    igraphdsaupd_(&ido, options->bmat, &options->n, options->which,
    		  &options->nev, &options->tol,
    		  resid, &options->ncv, v, &options->ldv,
    		  options->iparam, options->ipntr,
    		  workd, workl, &options->lworkl, &options->info);
#endif

    if (ido==-1 || ido==1) {
      igraph_real_t *from=workd+options->ipntr[0]-1;
      igraph_real_t *to=workd+options->ipntr[1]-1;
      if (fun(to, from, options->n, extra) != 0) {
	IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
      }
      
    } else {
      break;
    }
  }
  
  if (options->info == 1) {
    igraph_i_arpack_report_no_convergence(options);
  }
  if (options->info != 0) {
    IGRAPH_ERROR("ARPACK error", igraph_i_arpack_err_dsaupd(options->info));
  }
  
  options->ierr=0;
#ifdef HAVE_GFORTRAN
  igraphdseupd_(&rvec, all, select, d, v, &options->ldv,
		&options->sigma, options->bmat, &options->n,
		options->which, &options->nev, &options->tol,
		resid, &options->ncv, v, &options->ldv, options->iparam,
		options->ipntr, workd, workl, &options->lworkl,
		&options->ierr, /*howmny_len=*/ 1, /*bmat_len=*/ 1,
		/*which_len=*/ 2);
#else
  igraphdseupd_(&rvec, all, select, d, v, &options->ldv,
  		&options->sigma, options->bmat, &options->n,
  		options->which, &options->nev, &options->tol,
  		resid, &options->ncv, v, &options->ldv, options->iparam,
  		options->ipntr, workd, workl, &options->lworkl,
  		&options->ierr);
#endif

  if (options->ierr != 0) {
    IGRAPH_ERROR("ARPACK error", igraph_i_arpack_err_dseupd(options->ierr));
  }
  
  /* Save the result */
  
  options->noiter=options->iparam[2];
  options->nconv=options->iparam[4];
  options->numop=options->iparam[8];
  options->numopb=options->iparam[9];
  options->numreo=options->iparam[10];

  if (options->nconv < options->nev) {
    IGRAPH_WARNING("Not enough eigenvalues/vectors in symmetric ARPACK "
		   "solver");
  }

  if (values || vectors) { 
    IGRAPH_CHECK(igraph_arpack_rssort(values, vectors, options, d, v));
  }
  
  options->ldv=origldv;
  options->ncv=origncv;
  options->lworkl=origlworkl;
  options->which[0] = origwhich[0]; options->which[1] = origwhich[1];
  options->tol=origtol;
  options->nev=orignev;
    
  /* Clean up if needed */
  if (free_them) {
    igraph_Free(select);
    igraph_Free(ax);
    igraph_Free(resid);
    igraph_Free(d);
    igraph_Free(workd);
    igraph_Free(workl);
    igraph_Free(v);
    IGRAPH_FINALLY_CLEAN(7);
  }
  return 0;
}

/**
 * \function igraph_arpack_rnsolve
 * \brief ARPACK solver for non-symmetric matrices
 *
 * Please always consider calling \ref igraph_arpack_rssolve() if your
 * matrix is symmetric, it is much faster.
 * \ref igraph_arpack_rnsolve() for non-symmetric matrices.
 * </para><para>
 * Note that ARPACK is not called for 2x2 matrices as an exact algebraic
 * solution exists in these cases.
 *
 * \param fun Pointer to an \ref igraph_arpack_function_t object,
 *     the function that performs the matrix-vector multiplication.
 * \param extra An extra argument to be passed to \c fun.
 * \param options An \ref igraph_arpack_options_t object.
 * \param storage An \ref igraph_arpack_storage_t object, or a null
 *     pointer. In the latter case memory allocation and deallocation
 *     is performed automatically.
 * \param values If not a null pointer, then it should be a pointer to an
 *     initialized matrix. The (possibly complex) eigenvalues will be
 *     stored here. The matrix will have two columns, the first column
 *     contains the real, the second the imaginary parts of the
 *     eigenvalues.
 *     The matrix will be resized as needed.
 * \param vectors If not a null pointer, then it must be a pointer to
 *     an initialized matrix. The eigenvectors will be stored in the
 *     columns of the matrix. The matrix will be resized as needed.
 * \return Error code.
 *
 * Time complexity: depends on the matrix-vector
 * multiplication. Usually a small number of iterations is enough, so
 * if the matrix is sparse and the matrix-vector multiplication can be
 * done in O(n) time (the number of vertices), then the eigenvalues
 * are found in O(n) time as well.
 */

int igraph_arpack_rnsolve(igraph_arpack_function_t *fun, void *extra,
			  igraph_arpack_options_t *options,
			  igraph_arpack_storage_t *storage,
			  igraph_matrix_t *values, igraph_matrix_t *vectors) {

  igraph_real_t *v, *workl, *workd, *dr, *di, *resid, *workev;
  igraph_bool_t free_them=0;
  int *select, i;
  
  int ido=0;
  int rvec= vectors || storage ? 1 : 0;
  char *all="All";
  
  int origldv=options->ldv, origlworkl=options->lworkl,
    orignev=options->nev, origncv=options->ncv;
  char origwhich[2]={ options->which[0], options->which[1] };
  igraph_real_t origtol=options->tol;
  int d_size;

  /* Special case for 1x1 and 2x2 matrices */
  if (options->n == 1) {
    return igraph_i_arpack_rnsolve_1x1(fun, extra, options, values, vectors);
  } else if (options->n == 2) {
    return igraph_i_arpack_rnsolve_2x2(fun, extra, options, values, vectors);
  }

  /* Brush up options if needed */
  if (options->ldv == 0) { options->ldv=options->n; }
  if (options->ncv == 0) {
		igraph_i_arpack_auto_ncv(options);
  }
  if (options->lworkl == 0) { options->lworkl=3*options->ncv*(options->ncv+2); }
  if (options->which[0] == 'X') { options->which[0]='L'; options->which[1]='M'; }
  
  if (storage) {
    /* Storage provided */
    if (storage->maxn < options->n) {
      IGRAPH_ERROR("Not enough storage for ARPACK (`n')", IGRAPH_EINVAL);
    }
    if (storage->maxncv < options->ncv) {
      IGRAPH_ERROR("Not enough storage for ARPACK (`ncv')", IGRAPH_EINVAL);
    }
    if (storage->maxldv < options->ldv) {
      IGRAPH_ERROR("Not enough storage for ARPACK (`ldv')", IGRAPH_EINVAL);
    }

    v      = storage->v;
    workl  = storage->workl;
    workd  = storage->workd;
    workev = storage->workev;
    dr     = storage->d;
    di     = storage->di;
    d_size = options->n;
    resid  = storage->resid;
    select = storage->select;
    
  } else {
    /* Storage not provided */
    free_them=1;
    
#define CHECKMEM(x) \
    if (!x) { \
      IGRAPH_ERROR("Cannot allocate memory for ARPACK", IGRAPH_ENOMEM); \
    } \
    IGRAPH_FINALLY(igraph_free, x);

    v=igraph_Calloc(options->n * options->ncv, igraph_real_t); CHECKMEM(v);
    workl=igraph_Calloc(options->lworkl, igraph_real_t); CHECKMEM(workl);
    workd=igraph_Calloc(3*options->n, igraph_real_t); CHECKMEM(workd);
    d_size = 2*options->nev+1 > options->ncv ? 2*options->nev+1 : options->ncv;
    dr=igraph_Calloc(d_size, igraph_real_t); CHECKMEM(dr);
    di=igraph_Calloc(d_size, igraph_real_t); CHECKMEM(di);
    resid=igraph_Calloc(options->n, igraph_real_t); CHECKMEM(resid);
    select=igraph_Calloc(options->ncv, int); CHECKMEM(select);
    workev=igraph_Calloc(3*options->ncv, igraph_real_t); CHECKMEM(workev);
    
#undef CHECKMEM

  }
  
  /* Set final bits */
  options->iparam[0]=options->ishift;
  options->iparam[2]=options->mxiter;
  options->iparam[3]=options->nb;
  options->iparam[4]=0;
  options->iparam[6]=options->mode;
  options->info=options->start;
  if (options->start) {
    if (igraph_matrix_nrow(vectors) != options->n || igraph_matrix_ncol(vectors) != 1) {
      IGRAPH_ERROR("Invalid starting vector size", IGRAPH_EINVAL);
    }
    for (i=0; i<options->n; i++) {
      resid[i]=MATRIX(*vectors, i, 0);
    }
  }
  
  /* Ok, we have everything */
  while (1) {
#ifdef HAVE_GFORTRAN
    igraphdnaupd_(&ido, options->bmat, &options->n, options->which,
		  &options->nev, &options->tol,
		  resid, &options->ncv, v, &options->ldv,
		  options->iparam, options->ipntr,
		  workd, workl, &options->lworkl, &options->info,
		  /*bmat_len=*/ 1, /*which_len=*/ 2);

#else
    igraphdnaupd_(&ido, options->bmat, &options->n, options->which,
    		  &options->nev, &options->tol,
    		  resid, &options->ncv, v, &options->ldv,
    		  options->iparam, options->ipntr,
    		  workd, workl, &options->lworkl, &options->info);
#endif
    
    if (ido==-1 || ido==1) {
      igraph_real_t *from=workd+options->ipntr[0]-1;
      igraph_real_t *to=workd+options->ipntr[1]-1;
      if (fun(to, from, options->n, extra) != 0) {
	IGRAPH_ERROR("ARPACK error while evaluating matrix-vector product",
		     IGRAPH_ARPACK_PROD);
      }
      
    } else {
      break;
    }
  }

  if (options->info == 1) {
    igraph_i_arpack_report_no_convergence(options);
  }
  if (options->info != 0) {
    IGRAPH_ERROR("ARPACK error", igraph_i_arpack_err_dnaupd(options->info));
  }

  options->ierr=0;
#ifdef HAVE_GFORTRAN
  igraphdneupd_(&rvec, all, select, dr, di, v, &options->ldv,
		&options->sigma, &options->sigmai, workev, options->bmat,
		&options->n, options->which, &options->nev, &options->tol,
		resid, &options->ncv, v, &options->ldv, options->iparam,
		options->ipntr, workd, workl, &options->lworkl,
		&options->ierr, /*howmny_len=*/ 1, /*bmat_len=*/ 1,
		/*which_len=*/ 2);
#else
  igraphdneupd_(&rvec, all, select, dr, di, v, &options->ldv,
  		&options->sigma, &options->sigmai, workev, options->bmat,
  		&options->n, options->which, &options->nev, &options->tol,
  		resid, &options->ncv, v, &options->ldv, options->iparam,
  		options->ipntr, workd, workl, &options->lworkl,
  		&options->ierr);
#endif

  if (options->ierr != 0) {
    IGRAPH_ERROR("ARPACK error", igraph_i_arpack_err_dneupd(options->info));
  }

  /* Save the result */
  
  options->noiter=options->iparam[2];
  options->nconv=options->iparam[4];
  options->numop=options->iparam[8];
  options->numopb=options->iparam[9];
  options->numreo=options->iparam[10];

  if (options->nconv < options->nev) {
    IGRAPH_WARNING("Not enough eigenvalues/vectors in ARPACK "
		   "solver");
  }

  if (values || vectors) { 
    IGRAPH_CHECK(igraph_arpack_rnsort(values, vectors, options,
				      dr, di, v));
  }

  options->ldv=origldv;
  options->ncv=origncv;
  options->lworkl=origlworkl;
  options->which[0] = origwhich[0]; options->which[1] = origwhich[1];
  options->tol=origtol;
  options->nev=orignev;
  
  /* Clean up if needed */
  if (free_them) {
    igraph_Free(workev);
    igraph_Free(select);
    igraph_Free(resid);
    igraph_Free(di);
    igraph_Free(dr);
    igraph_Free(workd);
    igraph_Free(workl);
    igraph_Free(v);
    IGRAPH_FINALLY_CLEAN(8);
  }
  return 0;
}

/**
 * \function igraph_arpack_unpack_complex
 * \brief Make the result of the non-symmetric ARPACK solver more readable
 *
 * This function works on the output of \ref igraph_arpack_rnsolve and
 * brushes it up a bit: it only keeps \p nev eigenvalues/vectors and
 * every eigenvector is stored in two columns of the \p vectors
 * matrix.
 *
 * </para><para>
 * The output of the non-symmetric ARPACK solver is somewhat hard to
 * parse, as real eigenvectors occupy only one column in the matrix,
 * and the complex conjugate eigenvectors are not stored at all
 * (usually). The other problem is that the solver might return more
 * eigenvalues than requested. The common use of this function is to
 * call it directly after \ref igraph_arpack_rnsolve with its \p
 * vectors and \p values argument and \c options->nev as \p nev.
 * \param vectors The eigenvector matrix, as returned by \ref
 *   igraph_arpack_rnsolve. It will be resized, typically it will be
 *   larger.
 * \param values The eigenvalue matrix, as returned by \ref
 *   igraph_arpack_rnsolve. It will be resized, typically extra,
 *   unneeded rows (=eigenvalues) will be removed.
 * \param nev The number of eigenvalues/vectors to keep. Can be less
 *   or equal than the number originally requested from ARPACK.
 * \return Error code.
 *
 * Time complexity: linear in the number of elements in the \p vectors
 * matrix.
 */

int igraph_arpack_unpack_complex(igraph_matrix_t *vectors, igraph_matrix_t *values,
				 long int nev) {

  long int nodes=igraph_matrix_nrow(vectors);
  long int no_evs=igraph_matrix_nrow(values);
  long int i, j, k, wh;
  size_t colsize=(unsigned) nodes * sizeof(igraph_real_t);

  /* Error checks */
  if (nev < 0) {
    IGRAPH_ERROR("`nev' cannot be negative", IGRAPH_EINVAL);
  }
  if (nev > no_evs) {
    IGRAPH_ERROR("`nev' too large, we don't have that many in `values'",
		 IGRAPH_EINVAL);
  }

  IGRAPH_CHECK(igraph_matrix_resize(vectors, nodes, nev * 2));
  for (i=nev; i<igraph_matrix_nrow(values); i++) {
    IGRAPH_CHECK(igraph_matrix_remove_row(values, i));
  }
  
  /* Calculate where to start copying */
  for (i=0, j=0, wh=0; i<nev; i++) {
    if (MATRIX(*values,i,1) == 0) { /* TODO: == 0.0 ???? */
      /* real */
      j++;
    } else {
      /* complex */
      if (wh==0) { j+=2; wh=1-wh; }
    }
  }
  j--;

  /* if (j>=origcol) { */
  /*   IGRAPH_WARNING("Too few columns in `vectors', ARPACK results are likely wrong"); */
  /* } */

  /* We copy the j-th eigenvector to the (k-1)-th and k-th column */
  k=nev*2-1;

  for (i=nev-1; i>=0; i--) {
    if (MATRIX(*values,i,1)==0) {

      /* real */
      memset( &MATRIX(*vectors,0,k), 0, colsize);
      if (k-1 != j) {
	memcpy( &MATRIX(*vectors,0,k-1), &MATRIX(*vectors,0,j), colsize);
      }
      k-=2;
      j-=1;
    } else {
      /* complex */
      if (k!=j) {
	/* Separate copy required, otherwise 'from' and 'to' might
	   overlap */
	memcpy( &MATRIX(*vectors,0,k), &MATRIX(*vectors,0,j), colsize);
	memcpy( &MATRIX(*vectors,0,k-1), &MATRIX(*vectors,0,j-1), colsize);
      }
      if (i>1 && MATRIX(*values,i,1) != -MATRIX(*values,i-1,1)) {
	/* The next one is not a conjugate of this one */
	j-=2;
      } else {
	/* Conjugate */
	int l;
	for (l=0; l<nodes; l++) {
	  MATRIX(*vectors,l,k) = - MATRIX(*vectors,l,k);	 
	}
      }
      k-=2;
    }
  }

  return 0;
}
